This invention relates to the art of orthodontic appliances for applying protraction forces, and more particularly to a new and improved orthodontic appliance and method for treatment of Class III malocclusions (underbite) characterized by the lower teeth positioned anterior to or in front of the upper teeth when in centric occlusion.
The etiology involved in Class III malocclusions can range from a dental condition to a more severe skeletal malrelationship. In the Class III dental malocclusion, teeth are tipped towards the underbite position with normal maxillary and mandibular relationships. In skeletal malocclusion, the size or relative position of the maxilla and mandible varies. For example, the maxilla can be positioned posterior to an ideally positioned mandible. In another cases, the mandible is positioned anterior to an ideally positioned maxilla. Also, combinations of the previous examples are possible.
Treatment of a Class III malocclusion, characterized by open bite pattern, is difficult since such malocclusions result from many etiological factors. Skeletal open bite cases are usually associated with an increase in the vertical growth of the maxillary posterior dentoalveolar segments. The application of conventional reverse headgear, with the associated application of mesially directed force (below the center of resistance of maxillary dentition) tends to increase the anterior open bite. The intrusion of posterior teeth becomes more difficult with age, as mechanical treatment options are limited in adult patients. Orthognathic surgery may be indicated in adult patients with severe open bite and Class III skeletal patterns with retrognathic maxilla. Previous studies have shown the effects of protraction forces on the maxillary to be complex.
Used since the 1960s, commercially available reverse pull headgear designs typically have a metal or acrylic intra-oral portion attached to teeth, and an extra-oral portion that rests against the front of the face. These two components are attached with elastic bands that exert from 300 to 800 gram centimeters on the right and left sides. The direction of elastic traction is downward and forward at the level of the lips and not parallel to the Frankfurt horizontal plane. This produces a counter clockwise or upward and forward rotation of the maxilla while protracting. This rotation opens the bite, which is an undesirable side effect for certain dental relationships and facial types.
The most important factors to be considered in maxillary protraction are the point of force application and the direction of the force. Since the mandible is attached to the temporomandibular joint (TMJ), it is impossible to achieve a stable counterbalance force in reverse pull headgear by anchorage to the chin (due to movement of the mandible). Another drawback to using the chin to stabilize is the unknown effect orthopedic forces have on the TMJ and mandibular growth. In growing children, force application to the chin by reverse-pull headgear causes downward and backward rotation of mandible. Although the retrusive forces being applied by the chin cap can benefit a prognathic mandible, it may be detrimental to the TMJ as well as increase open bite tendencies.
The appliance and method of this invention distinguishes from conventional facemasks in avoiding upward rotation of the maxilla during protraction, and the invention is characterized by providing two independent adjustments that allow the invention to be customized to each patient.